There are warnings on both PFD's if the two PFD's differentiate (PIT, ROL, HDG). Those warnings were absent since the two PFD's went into declutter mode, because of the unusual "false" attitude on PFD1.
The simulator the 2 pilots regularly trained on, had those warnings present even in declutter mode. But not the real airplane, manufacturer was criticized by SHK.
The probable cause why they didn't check the standby attitude indicator, was that they were subjected to - 1G to +3G & many auditory and visual warnings.
Google translate from the final report by SHK:
"That none of the pilots of the situation verbally referring to standby attitude indicator can be explained by the two experiencing a complex situation because of the varying G loads and a great amount auditory and visual warnings. This has probably contributed further to cognitively
tunnel vision and focus on each PFD."
It's easy to be a hot-shot pilot in front your computer! But if you read the whole report it's more understandable why it happened.